1. Field of the Invention
The present invention relates to a microwave dielectric composition used for dielectric resonator, dielectric substrate, dielectric antenna and so on, in the field of mobile radio communication such as cellular phone, and more specifically relates to a new microwave dielectric composition having the excellent properties of quality factor, dielectric constant, and temperature coefficient.
2. Prior Art
Recent incrementation of communication information capacity promotes the use of high frequency band, and the communication by a microwave of 3GHz-30GHz is developed by remarkable force. The representative is cellular phone, and through miniaturization of high quality circuit components, the cellular phone is miniaturized/weight saved/multifunctionized and it becomes very popular quickly.
Transmitter and wave filter are the circuit elements transmitting and receiving microwave signal, respectively and the important components of communication equipment. Since dielectric ceramics are used as the circuit element which is transmitting and receiving microwave by resonance of microwave, said circuit element is called a microwave dielectric resonator. Today, popularization of cellular phone is caused by the inexpensive high quality microwave dielectric resonator.
Next three kinds of properties are required for the microwave dielectric resonator.    (1) Relative dielectric constant (εr) is large: when a wavelength of microwave in vacuum is λ0, the wavelength λ in said dielectric satisfies the equation λ=λ0/√εr. Since the size of resonator becomes 1/√εr, it is possible to miniaturise the resonators by using of the dielectric of large εr.    (2) Quality factor (Q·f: product of Q and f) is large:energy loss occurs when the microwave passes the dielectric, and this dielectric loss is expressed by tan δ. Since so called Q factor is given by Q=1/tan δ, the dielectric having small dielectric loss has large Q factor. This Q factor depends upon the resonance frequency f, and a relationship that the product of Q and f is constant, namely Q·f=const. is concluded.
This Q·f is called quality factor, and this factor is used in evaluation of dielectric loss because high quality dielectric has large value of Q·f.    (3) The temperature coefficient of resonance frequency (τf) is almost zero: it is desirable that the temperature coefficient (τf) of resonance frequency of a circuit system is zero or close to zero so that the resonance frequency does not vary with temperature.
Material development satisfying these conditions is going on, but the matter of fact is that materials development of dielectric possessing all three conditions is really difficult. Therefore, conventionally, materials of having high dielectric constant value are used in 1 GHz zone such as cellular phone or personal handyphone system (PHS), and composite perovskite materials of large Q factor are used in 10 GHz zone such as down converters of satellite broadcast. In other words, it is the present conditions that materials are used different according to frequency band.
The present inventors had studied zealously to develop a high quality microwave dielectric composition, and as a result, the inventors had conceived tungsten bronze type compositions mainly consisting of BaO, R2O3, 4TiO2. This microwave dielectric composition is expressed by Ba6−3xR8+2xTi18O54 (R is a rare earth element, x satisfies a range of 0.5≦x≦0.7.), and has been already published as Japanese Patent Open-Laid Publication No. 10-274005.
This tungsten bronze type composition is manufactured easily comparatively and suitable in the field of cellular phone required as mass production for a short term. However, so far as judging from three conditions of relative dielectric constant εr, quality factor Q·f, and temperature coefficient τf, the present is developmental stage of implementation of microwave dielectric material with better quality.
In such situation, the present inventors discovered excellent microwave dielectric properties in the vicinity of tungsten bronze type composition. It was found that the composition formula of this composition is expressed by BanLa4Ti3+nO12+3n (n=1,2 or 4).
While a further study, the present inventors found that this composition was already announced as “Microwave Dielectric Properties of Hexagonal Perovskites”(Materials Research Bulletin, Vol. 31(1996)pp. 431-437) by C. Vineis, P. K. Davies, T. Negas and S. Bell.
In this paper, C. Vineis et al. disclose the dielectric properties of composition expressed by composition formula of BaLa4Ti4O15 and Ba2La4Ti5O18.
These two compositions are understood as the cases of n=1 and 2 of said series of ceramic composition expressed by BanLa4Ti3+nO12+3n which present inventors systematically discovered.
As reported by C. Vineis et al., the homologous series compositions are microwave dielectric materials satisfying three conditions of relative dielectric constant εr, quality factor Q·f, and temperature coefficient τf of resonance frequency to some extent, which can be applied for microwave products of cellular phone and so on.
It is requested that the composition satisfies wide performance request by expanding such a Ba—La homologous composition of excellent characteristic expressed by BanLa4Ti3+nO12+3n (n satisfies n=1, 2, 4), to other elemental composition.
However, composition formula is limited for the present to the composition in the case of BanLa4Ti3+nO12+3n, besides n is natural number which satisfies n=1,2 or 4, with a composition of pure state, BaO, La2O3, and TiO2 must be measured appointed ratio at a manufacturing stage. In addition, it is called for an extremely severe manufacturing condition that mixture of impurity is removed and so, to produce said composition formula.
In addition, a strict manufacturing condition about an ingredient composition, a component ratio, and a purity degradation prevent inexpensive microwave dielectric unit price and cause of inducing a price rise of microwave dielectric resonator as a receiver transmitter. Therefore, a study to realize manufacturing with low unit price is necessary for the further spread of cellular phone and to establish the networking mainly on cellular phone.
Therefore, concerning said present invention, microwave dielectric composition and method for producing the same based on construction of BanLa4Ti3+nO12+3n, and changing the purity and composition formula to satisfy the wide characteristic request, at the same time, and also moderate strict manufacturing condition planed for reduction of manufacturing unit price.